1. Field of the Invention
The present invention relates to a low shrinkage polyamide fiber suitable for use as a yarn of a fabric for airbags, and an uncoated fabric for airbags produced using the same. The uncoated fabric for airbags is produced by a method comprising the steps of: (A) weaving a low shrinkage polyamide fiber having a dry heat shrinkage of 3-6% (190° C. for 15 minutes) into a grey fabric for airbags; (B) heat-shrinking the grey fabric by successively passing it through 3-10 aqueous baths, the temperature of each of which is 5-20° C. higher than that of the preceding aqueous bath; (C) additionally heat-shrinking the fabric from the aqueous baths by passing it through a steam heater; and (D) drying the fabric from the steam heater by passing it through a hot air drier.
The fabric produced using the low shrinkage polyamide fiber of the present invention has high tensile strength and tear strength, and excellent quality, and thus, is useful as a fabric for airbags.
2. Background of the Related Art
Recently, airbags become necessary to secure the safety of passengers and thus their mounting in vehicles is being gradually increased.
The requirements for the air bags include low permeability to ensure their smooth deployment in the event of a collision, high tenacity to prevent the damage and bursting of the airbags themselves, and flexibility to prevent the abrasion of the passenger's face upon their deployment, and the like. Recently, improvements in foldability and packability of airbag fabrics themselves, and a reduction in costs thereof, also become important factors.
Airbag fabrics are broadly divided into coated fabrics whose surface was coated with resin after weaving, and uncoated fabrics which are used intact after weaving. In view of the maintenance of low permeability as described above, the uncoated fabrics are generally advantageous for use in the airbags.
There have been many technologies to realize airbags which have excellent foldability and are packed at reduced volume without deteriorating the high tenacity and low permeability suitable for the airbags. For this purpose, for example, Japanese patent laid-open publication No. Heisei 1-41438 discloses producing an airbag fabric using a yarn strand made of fibers with a tenacity of at least 8.5 g/d and a monofilament fineness of less than 3 deniers. Although this publication discloses nothing regarding the difference between coated fabrics and uncoated fabrics, the airbag fabric disclosed in this publication are essentially a coated fabric whose surface was coated with elastomer, such as chloroprene rubber. If the technology disclosed in this publication is applied to the uncoated fabric, the tenacity and packability of the fabric could be surely satisfied, but the maintenance of low air permeability could not be sufficiently satisfied.
Furthermore, Japanese patent laid-open publication No. Heisei 4-201650 discloses a technology of producing an airbag fabric with excellent tenacity and foldability, in which the airbag fabric is produced using a polyamide multifilament made of a plurality of monofilaments each having a deformed cross-section with a deformation degree of 1.5-7.0, and a monofilament fineness of 1.0-12 deniers. However, if the technology disclosed in this publication is applied to the coated fabrics, the requirements for airbags will be satisfied, but it is applied to the uncoated fabrics, air permeability, particularly at seams, will remain as a problem to be solved.
Technologies regarding the uncoated fabrics include a method described in Japanese patent laid-open publication No. Heisei 7-252740. This publication discloses that flat cross section yarns having a flat ratio of at least 1.5 are used to produce an uncoated airbag fabric excellent in low permeability, foldability and packability. However, the uncoated airbag fabric has an air permeability of more than 0.3 cc/cm2/sec under low pressure (124 Pa), and thus, cannot sufficiently meet the recent requirement of low permeability.
Meanwhile, to meet the US standard FMVSS208 revised in 2000, making an inflator dual is being examined. Since this inflator has two-stage deployment, gas output at the second stage is greater than the output of the prior inflator. For this reason, there are required an air permeability lower than the prior art even under high pressure, and also a reduction in slippage between a sewing yarn and a fabric at the seams of airbags (hereinafter, referred to as the seam slippage).
From this point of view, uncoated fabrics produced using yarns with a total fineness of 300-400 dtex as disclosed in Japanese patent No. 2950954 hardly seem to have a sufficiently low seam slippage. Furthermore, Japanese patent laid-open publication No. Heisei 8-2359 discloses airbag fabrics having a weft/warp cover factor of 900-1400, which are characterized by a specified amount of oil remaining therein and specified slippage resistance. However, the airbag fabrics disclosed in this publication cannot also seem to be sufficient to meet the seam slippage.
U.S. Pat. No. 5,073,418 discloses a method where a cloth is produced using a yarn of less than 500 deniers and then calendered to reduce its permeability, and thus, to exhibit the effect of an improvement in its air tightness. However, this method is disadvantageous in that the tear strength of the cloth is reduced.
European patent publication No. 416483 discloses a heat-shrinkable or heat-shrunk, uncoated fabric for the production of airbags, in which the fabric is made of a synthetic filament yarn having a substantially symmetrical structure and a fineness of 300-400 dtex. However, a method for producing the fabric disclosed in this publication has a problem in that the tenacity of the synthetic filament yarn is rapidly lowered during a heat shrinkage process to reduce the tear strength of the fabric.
European patent publication No. 436950 discloses a method for the production of an industrial fabric which has a dense texture and does not need to be coated. In the disclosed method, the fabric made of a polyamide filament yarn having a hot-air shrinkage of 6-15% at 160° C. and at least substantially symmetrical structure is treated in an aqueous bath at a temperature of 60-140° C. However, this method has a problem in that the heat shrinkage of the synthetic filament yarn rapidly occurs in the aqueous bath of high temperature so that the quality of the fabric is lowered and the tear strength of the cloth is reduced.